Cryotropic hydrogels and their use as filters

ABSTRACT

There is provided a particle-filter comprising at least one hydrogel formed in aqueous solution at a temperature below 0° C. The filter can arrest particulates of smoke by physisorption on the solid surface of the gel or by chemisorption if the gel is suitably functionalized. The effectiveness of filter in arresting the solid particles and the pressure drop can be controlled by varying the porosity and the pore size distribution of the filter which in turn can be modulated by varying the monomer to water and monomer to cross-linker ratio in the pre-polymer solution. The filter is able to reduce the particle content of the smoke. There is further provided a cigarette comprising at least one particle-filter according as well as a method for the filtration of smoke.

TECHNICAL FIELD

The present invention relates to the field of cryotropic hydrogels andtheir use for filters. The present invention further relates to thedesign of smoke filters including a cigarette smoke filter usingcrosslinked porous materials. The invention further concerns a methodfor the filtration of smoke.

BACKGROUND

Cigarettes have always been a topic of conflict between the healthorganizations and the cigarette companies because of harmful andcarcinogenic compounds. In the 1950s, in the wake of rapidly growingscientific evidence that cigarettes cause lung cancer, companies triedto make special filters which would arrest the tar and other componentsfrom being inhaled with the smoke. Over the years several advancementshave been made in regard to filter design. In general, the filters havebeen designed using fibers made of thermoplastic polymeric materials.For example U.S. Pat. No. 4,059,121 describes a filter element made ofsolid fibers oriented in randomly manner. The diameter of these fibersrange up to 5 μm. While, this patent talks about a single filterelement, U.S. Pat. No. 4,149,550 describes a filter core made ofrandomly oriented fibers and a crust with densely packed fibers. Thusalong with the filtering efficiency, the strength of the porousstructure of the filter too has been targeted to be enhanced by thisprocedure. While these filters are made of single polymeric material,U.S. Pat. No. 4,579,130 proposes to use two different types ofcrystalline polymeric materials which are mixed and extruded to form thefibers. The patent further proposes to enhance the mechanical strengthof the filter by enhancing the adhesion between the fibers which arethermally and mechanically treated for this purpose. U.S. Pat. No.4,869,275 describes the design of an entangled web of very low bulkdensity made of plastic fibers of micron size diameter. Filtrationefficiency of as high as 80% is achieved in this filter. U.S. Pat. No.4,961,415 describes the use of non-woven thermoplastic polypropylenefibers, but here, in addition, the filter consists of regions made offused columnar structure which does not increase the pressure drop butimpart mechanical strength to the filter. Similarly U.S. Pat. No.5,538,019 envisages randomly oriented filaments formed into loop springsusing which a desired degree of firmness and pressure drop is achieved.Filter of a different kind is proposed in U.S. Pat. No. 5,586,987 whichdepicts the design of a thermoplastic elastomer-based fibrous nonwovenweb formed into a bag filter with an open, a close and inside surface. Abi-component sheath-core filter is envisaged by U.S. Pat. Nos. 5,633,082and 6,026,819 which describe the use of thermoplastic material likepolypropylene, polybutylene terephthalate and ethylene-vinyl acetatecopolymer. These polymers in different composition are used to form thecore and the sheath and a seamless continuity over these two regions.While the above patents desires essentially to achieve high filtrationefficiency while maintaining the mechanical integrity of the filter,these are several other patents which have been proposed to improve thequality of smoke being inhaled, by the addition into the matrix of thefilter of different components and molecules which either diffuse slowlyinto the smoke or arrest something toxic. For example, U.S. Pat. No.5,115,823 proposes to impart flavor to the smoke by a novel design whichcomprises of a filtering zone and a flavor enhancing zone containing theflavor. Similarly, U.S. Pat. Nos. 5,746,231 and 6,164,288 proposedispersing of a humectant like sodium pyroglutamate, chlorophyllin,vegetable oil etc. into the filtering material which captures themoisture in the smoke. This moisture then helps in wet filtering of thesmoke. U.S. Pat. No. 6,530,377 proposes to incorporate these moleculesinside microcapsules dispersed through the matrix of the filter. Besidethese humectants, there are other type of molecules that facilitateremoval of toxic chemicals have been proposed. For example, U.S. Pat.No. 6,792,953 describes the use of cellulose fibers impregnated withmetallic ions like copper and iron containing porphyrins. Similarly,U.S. Pat. No. 7,104,265 proposes addition of copper and ironphthalocyanine into the filter material. Beside the smoke quality, thedisposal of the filter material has also been a targeted issue. Forexample, there are patents which propose to design biodegradablecigarette filters; U.S. Pat. No. 5,817,159 proposes to design aninterpenetrating network made of two water soluble polymers.Importantly, the fibers for such filters are prepared without using anyorganic solvent. Similarly, U.S. Pat. No. 5,911,224 usespolyvinylalcohol as the fiber material which too is biodegradable.Since, polyvinylalcohol is hygroscopic, the patent describes also thespecial treatment procedure for carefully controlling the moisture ofthe fibers. While the above patents describe fibrous design of thefilter, U.S. Pat. No. 5,360,023 envisages filter made of web of paperincorporating a carbonaceous material. The papers arranged in a way toprovide longitudinally extending channels.

Details of preparation of cryogels have been dealt with in U.S. patentapplication Ser. Nos. 10/492,404 and 10/552,034. Previously cryogelshave been used for variety of applications e.g. for chromatographicseparation methods and as biomaterials. Another application has beenproposed by U.S. Pat. Nos. 5,288,503 and 5,460,715 which have describedrespectively cryogel diffusion barrier for release of therapeutic agentsand as a filtering medium for separating blood cells from plasma.

Whereas the existing filters have provided some hope for a safercigarette smoking, the particle count in the inhaled smoke intraditional cigarettes (Capstan, ITC brand) is still very high: 10⁹particles per centimeter cube of the smoke or higher. Thus there is roomfor improvement regarding cigarette filters.

SUMMARY

It is an object of the present invention to obviate at least some of thedisadvantages of the prior art and provide an improved particle-filteras well as a method for the filtration of smoke.

It is described to use a polymeric material known as cryogel which havebeen found to reduce the particle count significantly (10⁷ particles percentimeter cube) without increasing the pressure drop any significantly.

There is provided a particle-filter comprising at least one hydrogelformed in aqueous solution at a temperature below 0° C.

There is further provided a cigarette comprising at least oneparticle-filter as described above.

There is also provided a method for the filtration of smoke comprisingthe steps:

-   -   providing at least one filter comprising a macroporous hydrogel        formed in aqueous solution at a temperature below 0° C., and    -   filtering the smoke through the filter.

Further aspects and embodiments are defined in the appended claims,which are specifically incorporated herein by reference.

Advantages of the invention include that:

-   -   The cigarette filter is prepared without using any organic        solvent and the crosslinked network is not soluble in water in        contrast to many other types of filter materials.    -   The filter material does not cause any harm to the environment        or to the human body and is biodegradable.    -   The filter allows use in both dry condition or by incorporating        an amount of water in it. While water enhances the arresting        effect of the particulates that is the tar materials and other        components in the smoke, it increases the pressure drop. Since        the filter material is hydrophilic it contains the water in its        pores without allowing the water to wet the surface of its        container e.g. the paper wall of a conventional filter.    -   It is possible to achieve filtration via physical adsorption        onto the surface of the cryogel material or by specific chemical        reaction if the crosslinked network is suitably functionalized.    -   When the filtration occurs by physisorption, the filter can be        recycled after simple washing with water.    -   The crosslinked structure provides mechanical strength to the        filter without significantly increasing the pressure drop    -   The mechanical strength i.e. the deformability of the material        can be tuned by controlling the pore size distribution of the        filter.    -   The filter material is macroporous which helps in reducing the        pressure drop.    -   The macroporous structure is tuned by varying the water to        monomer ratio and the monomer to crosslinker ratio in the        pre-polymer solution.    -   The filter can be used as a substitute of the conventional        cigarette filters or can be used as an additional filter to the        native cigarette filter.

Definitions

Before the invention is disclosed and described in detail, it is to beunderstood that this invention is not limited to particular compounds,configurations, method steps, substrates, and materials disclosed hereinas such compounds, configurations, method steps, substrates, andmaterials may vary somewhat. It is also to be understood that theterminology employed herein is used for the purpose of describingparticular embodiments only and is not intended to be limiting since thescope of the present invention is limited only by the appended claimsand equivalents thereof.

It must be noted that, as used in this specification and the appendedclaims, the singular forms “a”, “an” and “the” include plural referentsunless the context clearly dictates otherwise.

If nothing else is defined, any terms and scientific terminology usedherein are intended to have the meanings commonly understood by those ofskill in the art to which this invention pertains.

“Cryogel” is used herein to denote a hydrogel formed in aqueous solutionat subzero temperatures.

“Hydrogel” is used herein to denote a network of crosslinkedwater-soluble polymer chains.

“A macroporous” material is a material containing pores with an averagediameter greater than 50 nm but not more than 50 μm, along withinterconnectivity between the pores.

“Smoke” is used herein to denote a collection of airborne solid andliquid particulates and gases emitted when a material undergoescombustion or pyrolysis. For instance smoke often comprises particlescomprising tar. Examples of substances in cigarette smoke include butare not limited to methane, acetylene, ethane, propene, chloromethane,propane, ethanol, acetaldehyde, butene, ethanol, acetonitrile, acrolein,acetone, acrylonitrile, isoprene, pentadiene, 2-butanone, hexane,benzene, dimethylfuran, pyridine, toluene, benzopyrine, napthalene,N-nitrosamine, chloro-biphenyls, phenols, 2-pentanone, 3-buten-2-one,3-pentanone, ethylbenzene, isobutyronitrile, isoprene, o-xylene,propionaldehyde, propionitrile, m-/p-xylene, styrene and toluene.

“A supermacroporous” material is a material comprising pores greaterthan 1000 nm (1 μm) along with interconnectivity between the pores.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects and advantages of this invention will be more apparentfrom the ensuing description when read in conjunction with theaccompanying drawings wherein:

FIG. 1 shows scanning electron microscope image of a typical cryogelsample. The image shows the presence of pores and the pore wall made ofthe polymeric material. The pore size is a function of the polymer andthe crosslinker concentration.

FIG. 2 shows schematic of the experimental set up for particle countanalysis in the smoke emitted by a burning cigarette. The cigarette 21is first attached to one end of a cylindrical holder 22 which containsalso a smoke filter 23. This filter is either a conventional filter orone pre-designed of cryogel material. The SMPS 25 (Scanning MobilityParticle Sizer Spectrophotometer) sucks in the smoke 24 at constant flowrate which brings in the tar and other content from the burningcigarette and analyze it to yield the particle size distribution of thesmoke 24.

FIG. 3 shows number density N_(p) of particles per cc volume of smoke isplotted against diameter D_(p) (nm) of particles. There is representedthe particle count in smoke released respectively by a single (D) anddouble (C) conventional cigarette filter, and the particle count insmoke released by John silver cigarette filter (E). There is furthershown particle count in smoke emitted by polyacrylamide cryogel filtersprepared by using 6% (A) and 7% (B) by weight of monomer respectively.The monomer to crosslinker ratio is maintained at 2:1 for both the gels.

FIG. 4 shows number density N_(p) of particles per cc volume of smoke isplotted against diameter D_(p) (nm) of particles. There is representedthe particle count in smoke released respectively by a single (H) anddouble (G) conventional cigarette filter, and the particle count insmoke released by John silver cigarette filter (I) as well as thatemitted by agarose-alginate (2:1) cryogel filter (F).

FIG. 5 shows number density N_(p) of particles per cc volume in smokereleased by the acrylamide cryogel filters prepared with varying monomerto cross-linker ratio plotted against the particle size D_(p) in nm.Here the monomer concentration is kept constant at 6%. J denotes a 3:1Cryogel, K denotes a 4:1 Cryogel.

FIG. 6 shows pressure drop (Pa) across the filters plotted against theflow rate (cc/min) of gas through the filters. There is representedpressure drop across a conventional cigarette filters Capstan (L) and anauxiliary cigarette filter-John Silver (N). There is also representedpressure drop offered by cryogel filters made of Agarose-alginate (2:1)(M) and Acrylamide (6%, 3:1) (O) respectively.

FIG. 7 shows Number density N_(p) of particles per cc volume of smoke isplotted against diameter D_(p) (nm) of particles. Comparison of particlereduction by agarose-alginate cryogel filters in wet (Q) and dry (P)condition.

DETAILED DESCRIPTION

In a first aspect there is provided a particle-filter comprising atleast one hydrogel formed in aqueous solution at a temperature below 0°C.

There is in one embodiment described a highly efficient particle-filtercomprising a cross-linked inter-connected macroporous hydrogel structureto reduce particulate components in mainstream cigarette smoke.

There in one embodiment provided a cigarette filter which has beenprepared by using crosslinked material e.g. “cryogel” synthesized usingacrylamide, or N-isopropylacrylamide monomers but not excluding others.There are also described filters synthesized by cryogel matrices ofsynthetic polymers like agarose and/or alginate. These materials areprepared without using any organic solvent and are biodegradable. Thefiltration process is achieved via physical adsorption onto the surfaceof the cryogel material or by specific chemical reaction if thecrosslinked network is suitably functionalized. The filter is recyclableand biodegradable. One advantage of the crosslinked structure is that itprovides mechanical strength to the filter without significantlyincreasing the pressure drop. Furthermore, both these parameters can betuned by controlling the pore size distribution. This cigarette filterhas been found to reduce the content of particles comprising tar in thecigarette smoke by several orders of magnitude from what is achieved bythe conventional filters used in cigarettes. The filter can be used inboth wet and dry condition. The macroporous nature and interconnectivitymakes these cryogel filters as highly efficient substitutes of theconventional cigarette filters or can be used as an additional filter tothe native cigarette filter.

The invention discloses the application of porous crosslinked materialslike cryogel as the base material for designing cigarette smoke filters.The cross linking polymerization is in one embodiment carried out insidea hollow solid cylinder in order to achieve a cylindrical structure(monolith) of the filter of specific dimensions to suit for applicationof use as filters.

In one embodiment the particle-filter is a cigarette filter.

In one embodiment the hydrogel of the particle-filter is insoluble inwater.

In one embodiment the hydrogel of the particle-filter comprises water.This has the advantage that water arrests particles, for exampleparticles comprising tar in cigarette smoke.

In one embodiment the hydrogel of the particle-filter is hydrophilic.This has the advantage that the filter contains water in its poreswithout allowing the water to wet the surface of its container, e.g. thepaper wall of a conventional filter.

In one embodiment the hydrogel of the particle-filter issupermacroporous.

In one embodiment the hydrogel of the particle-filter comprises at leastone material selected from the group consisting of poly-acrylamide,agarose, and alginate.

In one embodiment the hydrogel is formed at a temperature below −10° C.In one embodiment the hydrogel is formed at a temperature below thefreezing point of the solution comprising the monomer. In an alternativeembodiment the hydrogel is formed at a temperature 10 degrees Celsiusbelow the freezing point of the solution comprising the monomer.

In one embodiment the hydrogel of the particle filter is formed withoutusing any organic solvents. This has the advantage that it does notcause harm to the environment and the human body.

In a second aspect there is provided a cigarette comprising at least oneparticle-filter as described above.

In a third aspect there is provided a method for the filtration of smokecomprising the steps:

-   -   providing at least one filter comprising a macroporous hydrogel        formed in aqueous solution at a temperature below 0° C., and    -   filtering the smoke through the filter.

In one embodiment the at least one filter is recycled.

In one embodiment the at least one filter is washed with water, when itis recycled.

In one embodiment particulate matter in the smoke is reduced. In oneembodiment the reduction of particulate matter is more than 95 wt %. Inanother embodiment the reduction of particulate matter is more than 99wt %. In yet another embodiment the reduction of particulate matter ismore than 99.9 wt %. In an alternative embodiment the reduction ofparticulate matter removes essentially all particles.

In one embodiment the amount of at least one substance in the smoke isreduced, wherein the at least one substance is at least one substanceselected from the group consisting of methane, acetylene, ethane,propene, chloromethane, propane, ethanol, acetaldehyde, butene, ethanol,acetonitrile, acrolein, acetone, acrylonitrile, isoprene, pentadiene,2-butanone, hexane, benzene, dimethylfuran, pyridine, toluene,benzopyrine, napthalene, N-nitrosamine, chloro-biphenyls, phenols,2-pentanone, 3-buten-2-one, 3-pentanone, ethylbenzene, isobutyronitrile,isoprene, o-xylene, propionaldehyde, propionitrile, m-/p-xylene, styreneand toluene. In most cases the amount of several or all of the abovesubstances in smoke are reduced. In one embodiment all of the abovesubstances are reduced. The above substances are abundant in smoke suchas smoke from cigarettes.

In one embodiment the amount of the above at least one substance in thesmoke is reduced by more than 90 wt %. In a further embodiment theamount of the at least one substance in the smoke is reduced by morethan 95 wt %. In another embodiment the amount of the at least onesubstance in the smoke is reduced by more than 99 wt %. In anotherembodiment the amount of the at least one substance in the smoke isreduced by more than 99.9 wt %.

The hydrogel is in an alternative embodiment made from at least one typeof monomers or polymeric precursors selected from the group consistingof acrylamide, N-isopropylacrylamide, acrylonitrile, N-vinylcaprolactam,chitosan, gelatin, alginate, agarose and poly(vinylalcohol).

The cryogel is in one embodiment synthesized by using a polymerizationreaction in the water medium under freezing conditions which yield aspongy, elastic and supermacroporous material. Normally the freezingconditions are below 0° C.

In general the filters according to the present invention are useful forfiltering gases comprising particulate matter and substances in gasphase, for example the smoke generated by a cigarette, dust laden smokein many industrial situations e.g. mines, blast furnaces, cement plantsetc. Cryogel filters can be employed in disposable particulaterespirators for filtration of aerosols and fine particulates suspendedin air, which are associated with human disorders like asthma, lungcancer etc.

Other features and uses of the invention and their associated advantageswill be evident to a person skilled in the art upon reading thedescription and the examples.

It is to be understood that this invention is not limited to theparticular embodiments shown here. The following examples are providedfor illustrative purposes and are not intended to limit the scope of theinvention since the scope of the present invention is limited only bythe appended claims and equivalents thereof.

EXAMPLES

The cryogels of poly(acryl amide) [poly(AAm)] were synthesized byradical polymerization at −12° C. during 12 hours using monomers ofacryl amide (AAm) with N,N-ethylene bisacrylamide (MBAAm) ascross-linking agent. Similarly, agarose-alginate cryogels werefabricated by cross-linking agarose and alginate with the help ofglutaraldehyde at −12° C. for 16 hours.

When the polymerization occurs at sub zero temperature the water freezesand form nucleated crystals which grow and prevents the polymer reactionfrom occurring inside the crystals of water. After the polymerizationreaction is complete the gel is brought back to the normal temperatureat which the crystals melt resulting in the porous structure. The sizesof these pores depend on the polymer concentration, cross-linkerconcentration and the rate of cooling. The moisture content of thefilter is controlled by drying it to a desired extent.

The supermacroporous nature of these hydrogels was defined visually byscanning electron microcopy and ESEM (hydrogels in hydrated state). Thequantitative estimation was done by mercury porosimetry, uptake of waterand cyclohexane. The evaluated pore size is up to 200 μm.

The approach is to exploit the filtration properties of polymericmatrices called as “cryogels” to reduce the particulate count of smokeby using cryogels as auxiliary or substitute to conventional filters.

Cryogel filters have been used as auxiliary filters to observe reductionin total particle matter (TPM) in main stream cigarette smoke. Cigarettesmoke coming out of the cigarette butt having a single conventionalcigarette filter has about 10⁹ particles per centimeter cube (“D” inFIG. 3). These “cryogel filters” are macroporous materials having manyadvantages over the currently used cigarette filters as they removesignificantly higher number of particles comprising tar compared totraditional auxiliary filters like John Silver or Capstan's cigarettefilter used as auxiliary filter. As can be seen in FIG. 3, acrylamidecryogel (7% monomer, 2:1) as auxiliary filter reduces total particulatecount by a factor of about 10⁴ in comparison to TPM of cigarette havingits filter (“D” in FIG. 3). Even if an additional cigarette filter isused as auxiliary filter with the cigarette (“C”, FIG. 3) the TPMreduction is less in comparison to the reduction offered by cryogelfilters. Similarly 6% acrylamide cryogel also reduces particulate countby a factor of 10³ to 10² when compared to auxiliary Capstan cigarettefilter and auxiliary John Silver filter respectively (FIG. 3).

Agarose-alginate cryogel filter gives a particle reduction by a factorof 10⁴ in comparison to the particle reduction by traditional cigarettefilter, when both of them were used as auxiliary filters (FIG. 4).

Moreover, the synthesis protocol of the cryogel filters can be varied todesign cigarette filters with desired reduction in particulate count andoptimum draw resistance. As the polymer concentration of the cryogelincreases the particle count decreases e.g. the particle reductionoffered by acrylamide cryogel filter with 7% (total monomer)concentration is higher than that offered by 6% acrylamide cryogelfilter (FIG. 3). Similarly on decreasing the polymer to cross linkerratio in the cryogel the particle count passing through “cryogel filter”also decreases (FIG. 5). This change in porosity by varying the polymerconcentration and the cross linker to polymer ratio gives the cryogel ofdifferent porosity which changes the total particulate count (TPM) inthe smoke.

Along with particle count reduction, the draw resistance of the filtersis also an important property. To characterize these cryogel filters fordraw resistance, we measured the pressure drop across these filters whennitrogen gas was flowed through these cryogels at different flow ratesand compared them with the pressure drop of capstan cigarette filter andJohn silver filter. FIG. 6 shows that both acrylamide andagarose-alginate cryogel filters have comparable pressure drop as withcapstan cigarette filter, thus making them eligible candidates for useas cigarette filters.

1. A particle-filter comprising at least one hydrogel formed in aqueoussolution at a temperature below 0° C.
 2. The particle-filter accordingto claim 1, wherein said particle-filter is a cigarette filter.
 3. Theparticle-filter according to claim 1, wherein said hydrogel is insolublein water.
 4. The particle-filter according to claim 1, wherein saidhydrogel comprises water.
 5. The particle-filter according to claim 1,wherein said hydrogel is hydrophilic.
 6. The particle-filter accordingto claim 1, wherein said hydrogel is supermacroporous.
 7. Theparticle-filter according to claim 1, wherein said hydrogel comprises atleast one material selected from the group consisting ofpoly-acrylamide, agarose, and alginate.
 8. The particle-filter accordingto claim 1, wherein said hydrogel is formed at a temperature below −10°C.
 9. The particle-filter according to claim 1, wherein said hydrogel isformed without using any organic solvents.
 10. A cigarette comprising atleast one particle-filter according to claim
 1. 11. A method for thefiltration of smoke comprising the steps: providing at least one filtercomprising a macroporous hydrogel formed in aqueous solution at atemperature below 0° C., and filtering the smoke through the filter. 12.The method according to claim 11 , wherein said at least one filter isrecycled.
 13. The method according to claim 11, wherein said at leastone filter is washed with water.
 14. The method according to claim 11,wherein particulate matter in the smoke is reduced.
 15. The methodaccording to claim 14, wherein the amount of particulate matter isreduced by more than 99 wt %.
 16. The method according to claim 11,wherein the amount of at least one substance in the smoke is reduced,wherein the substance is at least one substance selected from the groupconsisting of methane, acetylene, ethane, propene, chloromethane,propane, ethanol, acetaldehyde, butene, ethanol, acetonitrile, acrolein,acetone, acrylonitrile, isoprene, pentadiene, 2-butanone, hexane,benzene, dimethylfuran, pyridine, toluene, benzopyrine, napthalene,N-nitrosamine, chloro-biphenyls, phenols, 2-pentanone, 3-buten-2-one,3-pentanone, ethylbenzene, isobutyronitrile, isoprene, o-xylene,propionaldehyde, propionitrile, m-/p-xylene, styrene and toluene. 17.The method according to claim 16, wherein the amount of the at least onesubstance in the smoke is reduced by more than 95 wt %.